A typical MRI system contains a large magnet having a bore therein for receiving patients to be imaged. The magnet, within the scope of this invention, could either be superconductive, resistive, or a permanent magnet. The typical system also includes gradient coils for varying the magnetic field in a known manner. Radio frequency (RF) coils are also provided within the magnet for "tipping" spins in the patient in order to generate the RF signal data used for producing images. Assoicated with the coils are gradient amplifiers and RF transmitters, receivers and signal processing systems.
The signals received responsive to the RF signal transmitted by the RF coils are extremely small. Therefore, MRI systems have a very low signal-to-noise (SNR) ratio. Especially because of the extremely low SNR, even the smallest amount of radio frequency interference affects the images adversely.
The radio frequency interference can penetrate shielded systems through the gradient cables by conduction and radiation. The source of the RF noise can be the gradient amplifiers and/or any other RF transmission, not necessarily related to the MRI system. In the latter case, the gradient cables operate as antennae; i.e., the radiated RF noise is received by the cables and conducted to the system. The RF noise, conducted through the gradient cables, is coupled from the gradient coils to the RF coils even when there is a decoupling RF mesh between the RF coils and the gradient coils, due to the limited capabilities of the RF mesh.
The above RFI penetration scenario is not the only possible one, and RF noise can be conducted into the vicinity of the RF coils through any other cables going into and out from an RF cage.
To prevent the RFI, it is typical to close the complete magnet system into an RFI shielded room. Such a room is ideally enclosed throughout by the use of walls, floors and ceilings that are covered with copper plating, for example. Where there is no room for a completely shielded room, then other forms of a Faraday cage are often used. These are often made up of a combination of copper plates and a copper mesh which completely surrounds the sensitive RF coil area, including the patient. This arrangement is often referred to as an RF cage. Then, the RF coils are within confines of an RFI cage and the magnet and gradient coils are outside of the confines of the RF cage.
Oft-times, part of the mesh serves as the RF coils to gradient coils decoupling device.
With the shielded room, the sensitive areas are surrounded by conductive surfaces; i.e., the RF cage. Currents that are induced by the external RFI electromagnetic fields are on the surface of the conductive cage. The induced currents produce electromagnetic fields which cancel the external RF fields inside the cage; thereby protecting whatever is in the interior of the cage from the external RF field.
Where RF cages or RF rooms are used, cables going into and from the cage are connected via feedthrough filters. There are commonly a few configurations of feedthrough filters such as, for example: L-type, T-type and x-type depending on the loading on each side of the specific cable. Basically, a feedthrough filter introduces high serial throughput impedance and low parallel impedance to the shield thus preventing the conductance of the interference through the shield.
It has been found that the abovesaid RF cage generally does not provide sufficient shielding due to the low signal provided by MRI systems and the signal's susceptibility to radio frequency interference noises.
Thus, those skilled in the art are still searching for an effective RF shield where there is insufficient room for a radio frequency interference-type room enclosed by copper plates.